Bevel not...beveling properly
The bevel appears distorted because the mesh has non-uniform scaling.

You probably scaled the object along a specific axis e.g. S > X. If you instruction had done this in edit mode, the scale doesn't change, but if one scales it in object mode, the scale will remain unapplied non-destructively. The visual appearance is the same. You can apply the scale in object mode, then return to edit mode and bevel.


Leave edit mode and go into object mode &#8633; Tab.
Press &#9096; CtrlA and choose Scale.
Go back into edit mode &#8633; Tab, the bevel will work as expected.